The process for fabricating a light-emitting device by severing into chips a nitride-based compound semiconductor wafer resulting from stacking an n-type layer, an active layer and a p-type layer on an insulating substrate, such as a sapphire substrate, comprises, as disclosed for example in JP-A HEI 05-343742, a step of etching a wafer to expose the n-type layer and form a trench shaped like a periphery of each chip, a step of polishing the substrate to decrease the thickness thereof, a step of inserting the diamond blade of a dicing saw into the trench and exposing the substrate, a step of inflicting a scribe line on the mark of dicing with a scriber, and a step of breaking the substrate with pressure to acquire chips. JP-A HEI 11-354841 discloses a severing process which comprises a step of exposing an n-type layer by etching and forming a trench shaped like a chip, a step of inserting the diamond blade of a dicing saw into the trench and exposing the substrate, a step of inflicting a scribing line with a scriber at a position corresponding to the dicing line from the second surface side of the substrate, and a step of breaking the substrate with pressure to acquire chips. These prior art references indicate that since the sapphire substrate and the nitride-based compound semiconductor layer are too hard to permit easy separation into chips by cleavage like GaAs and GaP, these substrates require decrease in thickness prior to separation into chips in order to be broken easily and they require dicing or scribing for imparting a stress concentrated part serving to facilitate breakage or forming a locally thinned portion serving to allow necessary breakage to occur at an expected position.
It has been known from long ago that the chip acquires an improved efficiency of light extraction when it is shaped like a reversed circular truncated cone. It is disclosed for example in Journal of Applied Physics, Vol. 35, 1964, page 1153, that when the light emitted from the emission region of a light-emitting device and injected through the lateral face of a chip is reflected in the direction of the emission observation face in the upper part of the chip and allowed to impinge substantially perpendicularly on the upper face of the chip, the light can be taken out of the chip without being reflected again on the emission observation face toward the interior of the chip. This technique can be effectively applied to the nitride-based compound semiconductor wafer using a sapphire substrate. JP-A HEI 06-244458, for example, discloses a technique which, in severing a flip-chip-type element from a chip, inserts a cut in the chip by dicing so as to separate a chip having an oblique lateral face.
The conventional processing devices have been used to adopt dicing saws using a diamond chip and scribers using a diamond chip. Recently, a device for forming a trench to be used in cutting chips by the use of a laser beam has been developed as disclosed, for example, in U.S. Pat. No. 6,413,839. The laser beam constitutes a processing technique which not only proves usable as a simple alternative means for the dicing saw and the scriber in popular use heretofore but also promises materialization of a processing method which has never been implemented by the conventional method. This technique, by controlling the beam diameter and the focal position of the laser beam and further the laser output and the duration of exposure, is enabled to vary the width and the depth of the trench to be formed. JP-A HEI 11-163403, for example, discloses a technique for forming a trench on the surface opposite the surface irradiated with the laser beam. Further, JP-B 2004-26766 discloses production of chips by a process which comprises a step of forming trenches shaped like chips by removing by etching part of the p-type layer and the light-emitting layer of a GaN epitaxial wafer produced by stacking an n-type GaN layer, a light-emitting layer and a p-type GaN layer on a substrate and used for a light-emitting device and consequently exposing the n-type layer, a step of polishing the substrate to decrease thickness, a step of irradiating the trenches from above with the laser beam and consequently forming trenches that expose the substrate and a step of breaking the substrate with pressure. Since a nitride-based compound semiconductor and a substrate for which an SiC and a sapphire is usually used are hard substances comparable to diamond, the processing performed thereon by the use of a dicing saw and a scriber has necessitated a highly advanced technique and consumed a long time. Since the use of the laser beam is capable of forming trenches narrower and deeper than those of the conventional processing devices, it can materialize the processing which has been accomplished by the conventional method only with difficulty.
The use of the laser-processing device allows forming of trenches deeper and stabler in shape than those obtained by the conventional dicing saws or scribers. Though the mechanism of breaking the sapphire substrate under pressure into chips while causing the stress exerted thereon to be concentrated on the bottom parts of the trenches is the same as in the conventional method, since the trenches are stable in shape, the end faces of the chips which form the parts for breaking the substrate can be severed with smooth facial shapes as compared with the conventional method. The use of the laser-processing device, however, results in bringing the luminance of emission of the LED chip to a lower level than the conventional method. Two causes are conceivable for this lowered luminance. One of them is the problem that the heat used during the laser processing may deteriorate the epitaxial layer. The other is the fact that since the end face of the chip is finished to be smoothed, the efficiency of light extraction in the end face of the chip is lowered as compared with the conventional method. The conventional method fails to obtain trenches in smoothed shape and consequently suffers broken faces to sustain a ragged contour and imparts an irregular jogging naturally to the end faces of the divided chips. In contrast, since the laser method is capable of forming trenches in a smooth and stable shape, it enables the broken faces to be finished in a smooth shape. The light from the emission region which is injected into the smooth shape tends to be reflected inside the chip and returned, with the result that the efficiency of light extraction through the end face of the chip will be degraded. This will be referred to as the first task concerning the processing.
There exists the second task concerning the processing which occurs in the conventional processing method and which also occurs in the laser method. Even when the laser method succeeds in forming deeper trenches, the substrate possibly will not be broken neatly in the cross sectional direction depending on the manner of exerting the stress during the division of the substrate into chips. That is, the stress has the possibility of warping the end faces of the chips.
The present inventor has made a diligent study with a view to solving the tasks mentioned above and has consequently arrived at this invention.
This invention has as an object thereof the provision of a method for fabricating a semiconductor light-emitting device which simultaneously solves the first task of enabling the light of emission to be extracted effectively even through a smooth surface and the second task of enabling the substrate to be broken into chips without warping the end faces of the chips during the course of breakage.